1. Field of the Invention
The invention relates to symbol generators particularly with regard to digital stroke generators with the vector, circle and character writing capabilities.
2. Description of the Prior Art
Stroke symbol generators utilizing analog components are known in the prior art. Such apparatus generally tends to be heavy, bulky and expensive compared to equipment utilizing digital components. Digital stroke generators are also known in the prior art but such display symbol generators generally do not provide vectors, circles and characters of high display quality. For example, in the generation of vectors utilizing the conventional sine-cosine technique the vectors generally tend to exhibit noise providing an indistinct appearance because of problems associated with the overflow signals from the multipliers or accumulators utilized in the X and Y display axes. Additionally for a given circuit complexity the average display writing rate is undesirably limited in the prior art displays. With regard to the generation of circular curves, the prior art digital displays do not achieve sufficiently fine control over the radius of the curvature of the curve to be generated. In such prior art apparatus, display of a circle of continuously variable radius tends to be erratic in appearance. The prior art apparatus is additionally limited in that when utilizing digital techniques, circle generation is achieved at a significantly lower writing rate than that utilized for vectors and characters thus providing non-uniformity in brightness with respect to the overall display.
Additionally, in prior art digital circle generators utilizing the sine-cosine technique the beam writing rate and hence the symbol intensity varies in accordance with the size of the circle. This occurs because with the sine-cosine method each circle is drawn in the same time irrespective of size.
Furthermore, in the prior art it was often necessary to utilize separate vector and circle generators resulting in not only excessive equipment but in registration and drift problems with respect to the elements of the display. The prior art symbol generators normally required a significant amount of control data resulting in excessive amounts of control circuitry.
The commercially procurable digital-to-analog converters utilized in digital symbol generators generally become extremely expensive in order to achieve rapid settling time between application thereto of data. This increase in expense has often resulted in a performance compromise with respect to display resolution and clock rate.